(a) Technical Field
The present invention relates to an operation control method of a fuel cell system. More particularly, the present invention relates to an operation control method of a fuel cell system, which increases the output of a fuel cell stack by detecting and overcoming the cause of a performance limit when the fuel cell stack reaches the performance limit while a fuel cell vehicle is being operated at high altitudes, thereby achieving improved stack performance and vehicle power performance.
(b) Background Art
A fuel cell is an energy conversion device that converts chemical energy stored in fuel into electrical energy via an electrochemical reaction, rather than changing the chemical energy into heat via combustion. Fuel cells may be used to supply industrial, domestic, and vehicular electric power, and also to supply electric power for small electric/electronic products and portable appliances.
The vehicular fuel cell currently being studied is a Polymer Electrolyte Membrane Fuel Cell (PEMFC) having a high electric power density. In the polymer electrolyte membrane fuel cell, a Membrane Electrode Assembly (MEA), which is the main constituent element thereof, is disposed at an innermost position. The membrane electrode assembly includes a solid polymer electrolyte membrane which is capable of moving hydrogen ions, and a cathode (i.e. air pole) and an anode (i.e. hydrogen pole) which are electrode layers on both surfaces of the electrolyte membrane and have a catalyst applied thereto to enable hydrogen and oxygen reactions.
In addition, a Gas Diffusion Layer (GDL) is stacked on the exterior portion of the membrane electrode assembly, i.e. the exterior portion at which the cathode and the anode are located. In turn, bipolar plates are disposed at the exterior of the gas diffusion layer and the bipolar plates form a flow field, through which reaction gas (e.g., hydrogen as fuel gas and oxygen or air as oxidant gas) is supplied, and through which cooling water passes.
For example, a gasket used to seal fluid is stacked to be interposed between the bipolar plates. After stacking a plurality of cells, end plates are coupled at an outermost position to support the cells therebetween. Since each unit cell generates a low voltage, scores to hundreds of cells are stacked in series to increase the voltage. Accordingly, the fuel cell manufactured in the form of a stack is used as a power generating device.
A fuel cell system to be applied to a fuel cell vehicle is composed of a fuel cell stack and devices that are configured to supply reaction gas. FIG. 1 is a view illustrating the configuration of a fuel cell system according to the related art. As illustrated in FIG. 1, the fuel cell system includes a fuel cell stack 10 configured to generate electricity from the electrochemical reaction of reaction gas, a hydrogen supply device 20 configured to supply hydrogen as fuel to the fuel cell stack 10, an air supply device 30 configured to supply air including oxygen to the fuel cell stack 10, a heat and water management system 40 configured to adjust the operating temperature of the fuel cell stack 10 and perform a water management function, and a fuel cell system controller (not illustrated) configured to operate the fuel cell system.
In the conventional fuel cell system, the hydrogen supply device 20 includes, for example, a hydrogen reservoir (e.g., a hydrogen tank) (not illustrated), a regulator (not illustrated), a hydrogen pressure adjusting valve 21, and a hydrogen recirculation device 22. The air supply device 30 includes, for example, an air blower (for low-pressure operation) or air compressor (for high-pressure operation) 32, a humidifier 33, and an air pressure adjusting valve 34. The heat and water management system includes, for example, an electric water pump (e.g., cooling water pump), a water tank, and a radiator, although not illustrated, as well as a water trap 41.
In the hydrogen supply device 20, high-pressure hydrogen supplied from the hydrogen tank is decompressed to a particular pressure in the regulator, prior to being supplied to the fuel cell stack 10. Accordingly, the decompressed hydrogen is supplied to the fuel cell stack 10 by a controlled supply amount via pressure control based on the operating conditions of the fuel cell stack 10. In other words, hydrogen, having passed through the regulator from the hydrogen tank, is supplied to the fuel cell stack 10 after being adjusted in pressure by the hydrogen pressure adjusting valve 21 at the inlet side of a stack hydrogen pole.
The hydrogen pressure adjusting valve 21 is adjusted to change pressure of the hydrogen, decompressed by the regulator, to be suitable for the stack operating conditions. The controller is configured to operate the hydrogen pressure adjusting valve 21 upon receiving feedback values from two hydrogen pressure sensors 25 and 26 which are installed respectively at the inlet and outlet sides of the stack hydrogen pole.
In addition, the hydrogen remaining after reaction inside the fuel cell stack 10 is discharged through the outlet of the stack hydrogen pole (anode), or is recirculated to the inlet of the stack hydrogen pole by the hydrogen recirculation device 22. The hydrogen recirculation device 22 is a device that increases the reliability of the hydrogen supply and improves the lifespan of the fuel cell. Although there are various recirculation methods, known exemplary methods include a method using an ejector 23, a blower, and both an ejector and a blower.
The hydrogen recirculation device 22 contributes to the reuse of hydrogen by recirculating unreacted hydrogen, remaining after used in the hydrogen pole (anode) of the fuel cell stack 10, to the hydrogen pole through a recirculation pipe 24. In addition, the hydrogen recirculation device 22 increases in the amount of impurities such as, for example, nitrogen, water, and vapor, which are directed to the hydrogen pole through the electrolyte membrane inside the stack of the fuel cell, and cause a reduced amount of hydrogen in the hydrogen pole, resulting in deterioration of reaction efficiency. Therefore, it is necessary to purge the impurities by opening a purge valve 27 at a predetermined period.
In other words, by installing the purge valve 27 for purging hydrogen to a pipe at the outlet side of the hydrogen pole of the fuel cell stack 10 to periodically discharge the hydrogen from the hydrogen pole, impurities such as, for example, nitrogen and water are also discharged and removed, and the rate of use of hydrogen increases.
Discharging the impurities from the fuel cell stack as described above advantageously increases the concentration of hydrogen, increases the rate of hydrogen use, and improves the diffusion and reactivity of gas. In addition, methods of operating the fuel cell system may be generally divided into a low-pressure operation method and a high-pressure operation method. In the respective operation methods, the operating pressure of the fuel cell stack is one of the factors having a predominant effect on the overall performance.
In a low-pressure operating fuel cell system, an air blower is generally used to supply low-pressure air to the air pole (cathode) of the stack. In a high-pressure operating fuel cell system, the air compressor 32 is used to supply higher pressure air to the air pole of the stack. In addition, in the high-pressure operating fuel cell system, the air having passed through a filter 31 is supplied to the air pole of the fuel cell stack 10 using the air compressor 32, and the outlet pressure of the air pole is controlled using a pressure adjustor at the rear end of the stack, i.e. the air pressure adjusting valve 34 mounted to a pipe at the outlet side of the air pole of the stack.
In general, to adjust the operating pressure of the fuel cell system, target values of inlet and outlet pressures of the hydrogen pole and the air pole are determined from an operating pressure control map based on the operating conditions of the fuel cell, and measured values of the hydrogen pressure sensors 25 and 26 and the air pressure sensors 35 and 36 are fed back to be adjusted to the target values for the inlet and outlet pressures of the hydrogen pole and the air pole.
Meanwhile, the cause of deterioration in the performance of a fuel cell system equipped in a fuel cell vehicle at high altitudes may be divided into (1) deterioration in the overall performance of the fuel cell stack due to the reduced operating pressure of an air pole and (2) a reduction in the maximum output of a fuel cell stack.
Among these causes, the deterioration of overall stack performance due to the reduced operating pressure of the air pole may be improved when the pressure of air supplied to the fuel cell stack is increased by supplying compressed air using a high-pressure operating system, i.e. an air compressor. However, the reduction of maximum output may be not be improved by simply applying the high-pressure operating system.
The cause of stack output limit due to the reduction of maximum output may be divided into (a) the insufficient flow rate of air and (b) the introduction of a minimum acceptable voltage due to the deterioration of stack performance. However, the cause may be the insufficient flow rate of air when the stack is good (e.g., no deterioration), and the cause may be the introduction of a minimum acceptable voltage due to the deterioration of stack performance when the deterioration of the stack has proceeded to some extent.
Therefore, there is the demand for an operation control method which is capable of increasing the maximum output of a fuel cell stack by positively determining the state of the stack while the fuel cell system is being operated at high altitudes.